1. Field of the Invention
This invention relates to the tethering of orbiting space vehicles, and particularly to a system for controlling the position of one space vehicle, e.g., along a local vertical relative to another.
2. General Description of the Prior Art
Tethered sub-satellites have been proposed for a variety of space applications. In a simple case, suggested for rescuing stranded astronauts, a buoy on a tetherline would simply be deployed with an initial velocity from the rescue vehicle to the astronaut, and then the tetherline would be reeled back after the astronaut had grabbed the buoy. A principal difficulty with this method is that slight variations in the initial velocity of the buoy with respect to the rescue vehicle, or errors in the tetherline tension, cause large errors in the final state of deployment or retrieval of the buoy.
Another proposed method of tethering two orbiting space vehicles together suggests that one of the vehicles be placed in a somewhat egg-shaped trajectory relative to the other and maintaining them in such position by periodically tugging on a tetherline which connects them. This approach was determined to be unattractive for flight because of the difficulty of precisely determining and controlling the tugging forces to be applied and of providing constant manned supervision which would be required to insure flight safety.
A third method calls for the deployment of a sub-satellite in an orbit below a main satellite wherein initial deployment would be accomplished with a drag balloon, which would then be destroyed, and final deployment effected by slowly unwinding the tetherline from a reel. While deployment in this manner would be operative, the realization of such a system would encounter the same problems discussed above, notably that of effecting sufficient stabilization of the sub-satellite and of eliminating deployment errors.